The present invention is directed to the field of automotive and truck tubing, and more particularly to the field of double walled tubing.
The use of double walled tubing, typically employed in the exhaust systems of vans, trucks, and the like, is highly valuable as a noise reduction element. Specifically, double walled tubing may be employed in large trucks, or other large equipment, between the engine and other noise reduction devices, such as the muffler and the like. When employed in this manner, the noise produced by the vehicle engine is significantly reduced due to the resonant properties, or lack thereof, of the double walled tubing. It is well known that a tube comprised of a single wall, when struck or otherwise imparted with energy, will vibrate or "ring" at a given resonant frequency or frequencies. When employed in large vehicles which inherently produce a great deal of noise energy, such single walled tubing can produce extremely undesirable sound levels.
On the other hand, it has been found that double walled tubing has significantly lower resonant properties. When struck or otherwise excited with wide band energy, rather than ring or resonate, the double walled pipe will dramatically attenuate the energy imparted thereto. For example, when dropped from several feet onto a hard surface, the double walled pipe will land with a dull "thud", as opposed to single walled pipe which would cause a great deal of noise.
Heretofore, the only technique for providing such double walled tubing is through the use of a tube mill process whereby two slabs of sheet metal, one right on top of the other, are formed into inner and outer tubes simultaneously in the tube mill. That is, the two sheets of metal are simultaneously formed into an inner and outer tube, and the seams of both the inner and outer tubes are welded simultaneously. This approach has many drawbacks, such as considerable expense, inconsistent quality, and abutting, welded seams. Since the seams on both the inner and outer tubes must be welded substantially simultaneously, the seams must necessarily be one on top of the other. It is therefore virtually impossible to prevent the welding of not only the seams of the inner and outer tubes to their respective counterparts, but also the welding of the inner seam to the outer seam. This is undesirable since axial movement of the inner tube within the outer tube is required during bending or otherwise shaping the double walled tubing. Since the inner and outer tubes are welded together in the prior art double walled tubing, such relative axial movement is severely limited, and the double walled tubing is not easily bent.